The present invention relates to a conductivity modulated semiconductor device which is used as a power switching device.
Recently, power MOSFETs have become available as power switching elements, but an element having a blocking voltage over 1000 V and a sufficiently low ON-state resistance has not appeared yet. This is because, in an ordinary power MOSFET, an ON-state resistance RON increases with the increase in blocking voltage VB. It is known that the following relation between them exists. EQU RON.varies.VB.sup.2.5
To solve such a problem, it has been considered to use a conductivity modulated (COM) FET as a power MOSFET. As shown in FIG. 1, this COMFET comprises a p.sup.+ -type drain region 1; an n.sup.- -type high resistance layer 2 formed on the drain region 1; p.sup.+ -type regions 3-1 and 3-2 selectively formed in the surface area of the high resistance layer 2; and n.sup.+ -type regions 4-1 and 4-2 formed in the surface area of the p.sup.+ -type regions 3-1 and 3-2. The surface regions of the p.sup.+ -type regions 3-1 and 3-2 between the high resistance layer 2 and the n.sup.+ -type regions 4-1 and 4-2 act as the channel regions. Namely, a gate electrode 5 is formed on a gate insulating film 6 over the surface regions of the high resistance layer 2 and p.sup.+ -type regions 3-1 and 3-2 which lie between the n-type regions 4-1 and 4-2. In addition, a first source electrode 7-1 is formed on the p.sup.+ - and n.sup.+ -type regions 3-1 and 4-1; a second source electrode 7-2 is formed on the p.sup.+ - and n.sup.+ -type regions 3-2 and 4-2; and a drain electrode 8 is formed under the p.sup.+ -type region 1. The structure of this COMFET is equivalent to a power MOSFET, called a vertical diffusion MOSFET, except that the drain region is formed by the p.sup.+ -type layer instead of the n.sup.+ -type layer.
The operation of this COMFET will now be described.
When the source electrodes 7-1 and 7-2 are grounded and a positive voltage is applied to the gate electrode and to the drain electrode 8, inverted layers, that is, channels are formed in the surface regions of the p.sup.+ -type regions 3-1 and 3-2 immediately beneath the gate electrode 5 in a similar manner as in the vertical DMOSFET. In this way, the COMFET is turned on similarly to the vertical DMOSFET. However, when the COMFET is turned on, holes are injected from the p.sup.+ -type drain region 1 to the n.sup.- -type high resistance layer 2 as well and are accumulated therein, thereby reducing the resistance value of the high resistance layer 2. This conductivity modulation effect makes it possible to increase the blocking voltage of the COMFET to a high value and to sufficiently decrease the ON-state resistance.
The COMFET shown in FIG. 1 has the drawback that the turn-off time is longer than that in the vertical DMOSFET. This is because it takes a long time for the carriers stored in the n.sup.- -type layer 2 to disappear.
FIG. 2 is a waveform diagram showing the operation of the COM switching device shown in FIG. 1. As will be obvious from this characteristic waveform diagram, when the COM switching device receives a gate voltage at the gate electrode 5 at time t.sub.N, channels are formed in the surface regions of the p.sup.+ -type regions 3-1 and 3-2; so that a drain current rapidly increases to a predetermined value. When the supply of this gate voltage is shut off at time t.sub.F, the drain current rapidly decreases to 0. The turn-off characteristic of the COM switching device includes first and second phases PH1 and PH2. In the first phase PH1, the channels in the p.sup.+ -type regions 3-1 and 3-2 disappear since the gate voltage becomes 0. Also, the electron currents flowing through these channels are shut out. Thus, a part of the drain current which is carried by elements is reduced instantaneously. In the second phase PH2, the carriers remaining in the n.sup.- -type layer 2 flow through the n.sup.- -type layer 2 and p.sup.+ -type region 1 due to the transistor action which is executed by the p.sup.+ -type regions 3-1 and 3-2, n.sup.- -type layer 2 and p.sup.+ -type region, and they are extinguished in accordance with the lifetime of those carriers. Thus, the drain current gradually decreases to 0.
In a conventional COM switching device whereby an impurity concentration and a thickness of the n.sup.- -type layer 2 are, respectively, 1.times.10.sup.14 (cm.sup.-3) and 40 to 50 (.mu.m), the turn-off time TOF is longer than 10 (.mu. sec).